This invention relates to optical systems in general and, more particularly, to an optical chopper with a high rate of focus dither.
Optical instruments employing optical choppers at their nominal focal, or image, planes often utilize a focus dither. Such dither can be introduced by several means including an intentional axial motion on some optical element in the optical train ahead of focus. Another technique for achieving similar results is the use of a chopper blade whose axis is not parallel to the axis of rotation. In such a scheme, the rotational axis is parallel to the instrument's optical axis and the field of view coincides with the area near the edge of the chopper. Chopper rotation then causes the blade to intercept the optical path at a sinusoidally varying position with respect to nominal focus. Such as system is depicted in FIG. 1. In this and other systems, an appropriate sensor is disposed at a point behind the blade and the output thereof, which is, thus, modulated at the chopper frequency, is utilized for control purposes.
This prior art technique illustrated in FIG. 1, along with other known prior art techniques, such as moving an optical element, suffer in that rapid modulation is impractical.
In view of this, it is the object of the present invention to provide an optical chopper which is not so limited as to rate of modulation and, thus, is not limited in its rate of focus dither.